Variable expansion force stent

ABSTRACT

A stent having varying outward radial force along its length. In use, the stent can provide greater force in vessel regions requiring greater force and less force in regions requiring less. In particular, more force is provided in the narrowed, center of a stenosis, while not applying too much force to the adjoining healthy tissue area. Greater stent expansion is provided in wider vessel geometries and less stent expansion in narrower regions. Varying force is achieved varying the number of elements, the density of elements, and the thickness of the elements.

FIELD OF THE INVENTION

[0001] The invention relates generally to medical devices. Morespecifically, the invention relates to stents for holding vessels suchas arteries open to flow.

BACKGROUND OF THE INVENTION

[0002] Stents are insertable medical devices used to maintain openingsfor fluid flow in areas that might otherwise close, hindering flow.Stents are used to prevent restenosis after Percutaneous TransluminalCatheter Angioplasty (PTCA), presenting outward radial force against apotentially rebounding vessel wall after balloon widening. Stents arealso used to hold open inflamed vessel walls that would otherwise beswollen shut, precluding flow. Stents can also be used to hold opensurgically made holes for drainage.

[0003] Stents are often tubular devices for insertion into tubularvessel regions. Balloon expandable stents require mounting over aballoon, positioning, and inflation of the balloon to expand the stentradially outward. Self-expanding stents expand into place whenunconstrained, without requiring assistance from a balloon. Aself-expanding stent is biased so as to expand upon release from thedelivery catheter.

[0004] A vessel having a stenosis may be modeled as an inwardlyprotruding arcuate addition of hardened material to a cylindrical vesselwall, where the stenosed region presents a somewhat rigid body attachedalong, and to, the elastic wall. The stenosis presents resistance to anyexpansion of the vessel in the region bridged by the stenosis. Stenosesvary in composition, for example, in the degree of calcification, andtherefore vary in properties as well.

[0005] The arcuate geometry of many stenoses present a variation inresistance along the vessel axis to stent outward radial force.Specifically, stenosed vessel resistance is often greatest toward themiddle, lessening toward the ends, with a rapid decrease at the start ofhealthy vessel tissue.

[0006] A conventional self-expanding stent optimally has a lengthgreater than the length of the stenosed region to be kept open. Currentstents present a substantially uniform outward radial force along theirlength. Currently, stents do not vary outward radial force to matchstenosis geometries or resistances. A constant force stent, withsufficient force to maintain an open channel within a stenosis, hasgreater force than necessary in the healthy vessel portion lying pastthe stenosis ends. The stent ends may thus flare outward, protrudinginto, and possibly irritating non-stenosed tissue.

[0007] Stenosis can occur in vessel regions having asymmetric geometrylying on either side of the stenosis. One example of this is the ostiumof a coronary artery, having a wide opening toward the aorta, converginginto a narrower coronary artery. A conventional stent placed in theostium would provide substantially uniform outward force over anon-uniform vessel diameter. If this force is properly matched for thenarrower vessel opening, it is likely less than optimal for the widerregion.

[0008] What would be desirable, and has not heretofore been provided, isa stent capable of providing sufficient force to keep a vessel openwithin a rebounding stenosis, while providing only necessary forceagainst healthy, non-stenosed vessel regions. What also has not beenprovided is a stent providing necessary, but only necessary force alonga stenosis in a vessel region having non-uniform vessel diameter oneither side of the stenosis.

SUMMARY OF THE INVENTION

[0009] The present invention includes a self-expanding stent having atubular shaped structure, where the outward radial force varies withlongitudinal position along the length of the stent. In one embodiment,the force is greater in the center and lesser at both ends. Such a stentis suitable for placement in a stenosed vessel region. In anotherembodiment, the force is less at one end, greater at the middle, andgreater still at the opposite end. Such a stent is suitable forplacement in a stenosed and narrowing vessel region, including placementnear a coronary ostium.

[0010] One stent has a structure formed of shape memory material. In oneembodiment, the stent is constructed of a Nickel-Titanium alloy.

[0011] The stent structure in a preferred embodiment includes a helixformed of a wire having the helix turns spaced more closely togethertoward the center than at the ends. The helix is biased to expand inouter diameter and contract in length after having been stretchedaxially and released. In an alternate embodiment, the helix turnsincrease in spacing from one end to the opposite end. In anotherembodiment, interwoven or intertwined wires form the tubular structure,with the number of wires being greater per unit length toward the centerthan at the ends. The interwoven wires can be metallic wire. The wirescan resemble spirals or helices after having been wound to the tubularstent shape. In yet another embodiment, the number of wires increasefrom one end to the opposite end.

[0012] One stent achieves a variation in radial force by including inthe stent structure elements which intersect at junctions having morematerial in regions requiring more radial force and less material inregions requiring less radial force. The amount of junction material canbe varied by varying the size of the junction area. In a preferredembodiment, the stent structure is formed by laser cutting a Nitinoltube, leaving a greater strut dimension in regions requiring greateroutward radial force.

[0013] In yet another embodiment, the stent structure includes a seriesof wire springs having a “zig-zag” shape which each radially encircle atubular section. The springs are interconnected longitudinally. Therequired outward radial force can be varied by varying the stent wallthickness in this and other embodiments. In one embodiment, stentregions requiring greater radial force have thicker walls than regionsrequiring less force.

[0014] Stents made in accordance with the present invention can providean outward radial force more closely matching the local forcerequirements. In particular, the stents provide greater force only whererequired in a stenosis center, without providing too much force in theregion of healthy tissue. The stents provide an expanded geometry moreclosely tailored to the requirements of a narrowing vessel region,providing greater expansion in wider regions and less expansion innarrower regions.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 is a fragmentary longitudinal cross-sectional view of astenosed vessel region;

[0016]FIG. 2 is a fragmentary cross-sectional view of a stenosed vesselregion with a conventional stent in place;

[0017]FIG. 3 is a plot of force versus length for the conventional stentof FIG. 2;

[0018]FIG. 4 is a fragmentary longitudinal cross-sectional view of astenosis in a narrowing vessel region;

[0019]FIG. 5 is a plot of force versus length of an improved stent forplacement in FIG. 1;

[0020]FIG. 6 is a plot of force versus length of an improved stent forplacement in FIG. 4;

[0021]FIG. 7 is a side view of a self expanding stent having more wiresper unit length at longitudinal center;

[0022]FIG. 8 is a side view of a self-expanding stent coil more-closelyspaced toward center;

[0023]FIG. 9 is a side view of a self-expanding stent having thickerelements toward longitudinal center;

[0024]FIG. 10 is an end view of the stent of FIG. 9;

[0025]FIG. 11 is a wafer view of the stent of FIG. 9;

[0026]FIG. 12 is a longitudinal profile of an alternate embodiment ofthe invention in which the diameter is non-uniform along the stentlength;

[0027]FIG. 13 is an enlarged view of element junctions in aself-expanding stent;

[0028]FIG. 14 is an enlarged view of an element junction in theself-expanding stent of FIG. 13;

[0029]FIG. 15 is an enlarged view of an element junction of aself-expanding stent;

[0030]FIG. 16 is a side view of a self-expanding stent having a greaterdensity of elements toward one end; and

[0031]FIG. 17 is a side view of a self-expanding stent having moreclosely spaced elements toward one end.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0032]FIG. 1 illustrates a stenosis 30, forming narrowed region 34, in avessel 31 within vessel wall 32. Adjacent to stenosis 30 is a healthyvessel region 36. FIG. 2 illustrates a conventional stent 40 in placeacross stenosis 30, out of the blood flow channel as indicated at 44.Stent 40 includes a stent end 44, shown angling into healthy vessel area36 at 38. Stent 40 as shown, has sufficient force to keep vessel 30 openagainst the rebound force of stenosis 30, and has more force thanrequired at stent end 42, resulting in stent 40 angling into the healthyvessel wall at 38. FIG. 3 illustrates an idealized plot 50 of outwardradial force, F, against stent length, L, for a conventional stent suchas that illustrated in FIG. 2. As shown, the force is substantiallyconstant over the length.

[0033]FIG. 4 illustrates a narrowing vessel 52 having a wide region 56,a narrowed region 58, and a stenosis 54. The narrowing vessel of FIG. 4illustrates the geometry as found in an ostium such as the left coronaryostium, where blood from the aorta flows into the left coronary artery.A stent with sufficient force to hold open wide region 56 would havegreater force than necessary to hold open narrowed region 58. A stenthaving the outward radial force axial distribution of FIG. 3, would haveinsufficient force at wide region 56 and greater than required force atnarrowed region 58.

[0034]FIG. 5 illustrates a plot 60 of outward radial force F along stentlength L for one stent embodying the present invention. The stent hasgreater force in a middle region 62 than at end regions 64 and 65. Astent having the force curve of FIG. 5 is suitable for bridging astenosis as illustrated in FIG. 1, while preventing the stent fromangling into healthy tissue as show in FIG. 2 at 38. FIG. 6 illustratesa plot 66 of outward radial force F along stent length L for anotherstent embodying the present invention. The stent has a greater force inend region 68 than at the opposite end region 70. A stent having theforce curve of FIG. 6 is suitable for bridging the stenosis asillustrated in FIG. 4, having sufficient force to hold open vessel wideregion 56 and less force in vessel narrow region 58, where less isrequired.

[0035]FIG. 7 illustrates a preferred embodiment of the inventionproducing a force distribution as illustrated in FIG. 5. Self-expandingstent 80 includes numerous resilient wires 82, interwoven as indicatedat 88. In use, stent 80 is drawn longitudinally which increases thelength and decreases the diameter. Stent 80 is inserted into the distalend of the delivery catheter, advanced to a stenosis to be crossed, andforced out of the delivery catheter distal end. Upon exiting the tube,stent 80 expands radially and shortens axially, pushing against thestenosis and vessel walls.

[0036] Stent 80 includes a middle region 84 and end regions 86 and 87.Stent 80 wires 82 are biased to resume the unconstrained state, which iswider and shorter than the constrained stent shape in the tube. Theamount of outward radial force exerted per unit length of stent isgreater in regions having a greater density of wires per unit length. Asillustrated in FIG. 7, stent 80 has a greater number of wires per unitlength in center region 84 than in end regions 86 and 87. Thus, stent 80has a greater outward radial force in center region 84 than in endregions 86 and 87. The greater number of wires per unit length in oneembodiment is the result of forming wires, which run the entire stentlength, more closely together toward stent center. In anotherembodiment, the greater number of wires is the result of adding morewires which only run in the center region of the stent.

[0037]FIG. 8 illustrates another embodiment of the invention inself-expanding stent 90, having a middle region 94 and end regions 96and 97. Stent 90 is formed of a single, spirally wound wire 92, forminga helix 98. A preferred embodiment utilizes Nitinol material for wire92. Helix 98 has a distance between helix turns as indicated at 99.Distance 99 varies with longitudinal position, being greater in middleregion 94 and less in end regions 96 and 97. Wire 92 is formed as aspring, biased to resume its unconstrained shape when released, afterhaving been stretched axially. The amount of outward radial forceexerted is greater in regions having more wire elements per unit length,which, in stent 90, is achieved by having less space 99 between helixturns. Thus, stent 90 has a greater outward radial force in centerregion 94 than in end regions 96 and 97.

[0038]FIG. 9 illustrates still another embodiment of the invention instent 100, having a middle region 104 and end regions 106 and 107. Stent100 has a tubular shape formed of a wire 102, which is shaped intoseveral springs 108 having a zig-zag pattern, each spring 108 radiallyencircling a segment of stent 100, as indicated in FIG. 10. Referringagain to FIG. 9, springs 108 are longitudinally interconnected withsegments 109. Springs 108 and segments 109 in one embodiment are formedusing standard wire bending jigs and techniques, including brazingsegments 109 to springs 108. A preferred material for constructing stent100 is Nitinol. In another embodiment, springs and segments are formedby laser cutting a continuous-walled metallic tube, leaving only springs108 and segments 109.

[0039]FIG. 11 illustrates a wafer section in elevation taken along 11-11in FIG. 10. Wire elements 102 are illustrated in cross section in middleregion 104 and end region 107. The element thickness in width and/orlength in end region 107, indicated at 101, is less than the elementthickness in middle region 104, indicated at 103. Middle elements havingthickness 103 can provide greater outward radial force than end elementshaving relatively lesser thickness 101. The radial expansive force canalso be varied by varying the frequency and/or amplitude of the zig-zagpattern.

[0040]FIG. 12 illustrates, in highly diagrammatic form, a phantom lineprofile of another embodiment of the invention. A profile of stent 110is shown in phantom, having a middle region 114 and end regions 116 and117. Stent 110 is formed, at least in part, from a shape memorymaterial. In the preferred embodiment, stent 110 is formed of Nitinol.Shape memory materials can be annealed into a first shape, heated,thereby setting the material structure, cooled, and deformed into asecond shape. The first shape has an average outside diameter greaterthan the second. The material returns to the first, remembered shape ata phase transition temperature specific to the material composition.

[0041]FIG. 12 illustrates the stent shape to be remembered upon reachingbody temperature. Stent 110 has a middle outside diameter 113 and endoutside diameter 111, where the middle outside diameter is greater thanthe end outside diameter. Stent 110 can be compressed to fit within thedelivery catheter, the delivery catheter advanced to a stenosis, and thestent pushed out the delivery catheter distal end. Stent 110 then beginsresuming the remember shape of FIG. 12. The stenosed region typicallyhas the arcuate shape of FIG. 1. As stent middle outside diameter 113 isgreater than end outside diameter 111, and the vessel middle insidediameter is typically less than the vessel end inside diameters, stent110 can provide greater force in applying middle stent region 114against middle vessel walls than in applying end stent regions 117 and116 against the end vessel walls.

[0042]FIG. 13 illustrates another embodiment of the invention. Inparticular, FIG. 13 illustrates a tubular stent structure formed ofelements meeting at junctions, where the junction size can be variedover the length of the stent. Stent 120 is shown having a structure 122including elements 124. Elements 124 intersect each other at junction130 as illustrated in detail in FIG. 14. FIG. 15 illustrates a junctionhaving a greater amount of material than the junction in FIG. 14. In theembodiment of FIG. 15, junction 132 has a greater surface area thanjunction 130. Junctions having more material have greater capacity toprovide radial outward force than junctions having less material. Oneembodiment of the invention has elements meeting or intersecting atjunctions, where the junctions have more material in the tube middleregion and less material in the tube end regions. In a preferredembodiment, the junctions are formed by laser cutting a Nitinol tubematerial.

[0043] In use, the tube can be compressed to fit within the deliverycatheter, advanced to the stenosis, and pushed distally from thedelivery catheter distal end. As the tube regains its uncompressedshape, areas having a greater amount of material at the junctions areable to exert greater outward radial force.

[0044]FIG. 16 illustrates an embodiment of the invention suitable foruse across stenoses in narrowing vessel regions, such as the leftcoronary ostium. Stent 140 has a first end region 147 and a second,opposite end region 146. Stent 140 is similar to stent 80 in FIG. 7. Thestent tube includes wires 142 which are wound around the stent and canbe interwoven. As illustrated in FIG. 16, wires 142 have a greaterdensity per stent unit length at second end region 146 than in first endregion 147. This enables second end region 146 to provide greateroutward radial force than first end region 147. Thus, first end region147 can be suitably matched for narrow vessel region 58, with second endregion 146 matched for wide vessel region 56.

[0045]FIG. 17 illustrates another embodiment of the invention suitablefor use across a stenosed, narrowing vessel region. Stent 150 extendsfrom a first end region 157 to a second end region 156. Stent 150 issimilar in construction to stent 90 in FIG. 8, including wires 152formed into a helix or spiral 158. Helix turns are spaced a distance 159apart. As illustrated in FIG. 17, helix turns are spaced further apartat first end region 157 than at second end region 156. This spacingallows stent 150 to provide greater outward radial force at second endregion 156 than at first end region 157.

[0046]FIGS. 16 and 17 illustrate two embodiments having greater radialforce at one end than the other. This property can be produced usingother structures. Another embodiment having this property is similar toa longitudinal half of FIG. 9, having a greater element thickness at oneend than the other. Yet another embodiment is similar to a longitudinalhalf of FIG. 12, having a greater outside diameter at one end than theother.

[0047] Stents providing greater outward radial force at one end thananother, as in the embodiments of FIGS. 16 and 17, allow a stent to beplaced across a stenosis in a narrowing vessel region as illustrated inFIG. 4. The stent end having a greater radial force can expand into thewider vessel region, while the stent end having lesser radial force canexpand to the narrower vessel region wall, but with less force than ifrequired to expand as far as the stent end in the wider vessel region.This can lessen unneeded force on the vessel wall while still holdingthe vessel open and keeping the stent substantially out of the vesselflow path.

[0048] The present invention provides a stent having a radial forcevaried along stent length. The stent has been described, in use, asbridging stenosed vessel regions for illustrative purposes. Another useis maintaining open channels through inflamed or otherwise restrictedbody conduits. Stents used for other purposes are explicitly within thescope of the invention.

[0049] It should be noted that although self-expanding stents have beenshown herein to illustrate the present invention, so called balloonexpandable stents can also include the variable expansion force featureas described herein. In the case of balloon expandable stents, however,these forces in general will be less than are necessary to expand thestent and thus the balloon will be used as known to those skilled in theart to complete the expansion of the stent. These balloon expandablestents may be advantageously deployed in bending areas of a vessels suchas at an ostium where a stent having thus rigid or heavy members isdesirable to enhance the flexibility of the stent. It should beunderstood therefore, that balloon expandable stents are also within thescope of the present invention.

[0050] Numerous characteristics and advantages of the invention coveredby this document have been set forth in the foregoing description. Itwill be understood, however, that this disclosure is, in many respects,only illustrative. Changes may be made in details, particularly inmatters of shape, size, and arrangement of parts without exceeding thescope of the invention. The inventions's scope is, of course, defined inthe language in which the appended claims are expressed.

What is claimed is:
 1. A stent having a length and a radius along said length comprising: a tubular shaped structure, said structure having a radially outward biased force, said force being varied along said length, said tubular structure having a first end region, a middle region, and a second end region, wherein said force is weakest in said first end region, stronger in said middle region than in said first region, and stronger in said second end region than in said middle region.
 2. A stent having a length and a radius along said length comprising: a tubular shaped structure, said structure having a means for exerting a radially outward force, said force being varied along said length, said tubular structure having a middle region, a first end region and a second end region, wherein said means for exerting a force includes a number of helices, said number being at least one, each of said helices including a plurality of turns, said turns having an axial spacing therebetween, said spacing being less in said middle region and greater in said first and second end regions.
 3. A stent as recited in claim 2, said stent having a length, wherein said number of helices is more than one, said helices being intertwined along said length.
 4. A stent having a length and a radius along said length comprising: a tubular shaped structure, said structure having a means for exerting a radially outward force, said force being varied along said length, said tubular structure including a plurality of intersecting elements, said intersecting elements intersecting at a junction including an amount of material, said junction material amount being greater in said middle region than in said end regions.
 5. A stent having a length comprising: a tubular shaped structure, said structure formed of a shape memory material, said structure having a first position and a second position, said first position having an average outside diameter less than said second position, said second position having an outside diameter along said length, said second position outside diameter varying along said length.
 6. A stent as recited in claim 5, said stent having a middle region, a first end region and a second end region, wherein said second position outside diameter is greater in said middle region than in said end regions.
 7. A stent as recited in claim 5, said stent having a first end region, a middle region, and a second end region, wherein said second position outside diameter is smaller in said first end region, larger in said middle region than in said first region, and larger in said second end region than in said middle region. 